1. Field of the Invention
The present invention relates generally to polarization-state-altering cube corners, and more particularly to methods and apparatuses for cube corners which rotate all linear polarizations by 90°, called linear polarization orthogonalization.
2. Discussion of the Background
A variety of applications, such as detection of objects passing a sensor (objects on a conveyor belt, cars at a toll booth, etc), and distance measurement, require an optical system as follows. The system is illuminated, by a lamp, laser or other optics, henceforth called the “source”. Some fraction of the irradiance from this source is transmitted through a beam splitter, and then over some path to a cube-corner retro-reflector. The irradiance returns to the beam splitter along the same path, and the beam is partially reflected by the beam splitter. This reflected irradiance then exits the system, either by absorption by a detector, or by passing into other optics. This exit is henceforth referred to as the “detector”.
On some systems, a non-polarizing beam splitter is used. Those skilled in the art will recognize that two passes through such a beam splitter results in a maximum of 25% of the source irradiance present at the detector. An illustration of such a system incorporating a non-polarizing beam splitter is shown in FIG. 1. FIG. 1 illustrates irradiance entering the system 1. The irradiance is split by the non-polarizing beam splitter 2. Fifty percent of the irradiance exits the system at 3 while the other fifty percent enters the cube-corner 4. The cube corner 4 then returns the irradiance and half the irradiance returns to the source 5 (25% of the original irradiance) and the other half reaches the system exit 6. Other systems use a polarizing beam splitter. The efficiency of these systems varies with the polarization properties of the retro-reflector. Some systems use a dielectric total internal reflection cube-corner, whose polarization properties depend on the refractive index, but to first order are depolarizers, due to the large polarization changes which are different in each of the hexads. These systems can have 50% of the source irradiance present at the detector. Such a system is shown in FIG. 2. FIG. 2 illustrates p-polarized irradiance entering the system 7. Because the p-polarized irradiance enters a polarized beam splitter (“PBS”) 8, the entire entering irradiance exits the PBS 9 and enters the cube corner 10. The polarized irradiance then exits the cube corner 10 as depolarized irradiance 11. The p-polarized portion of the irradiance is then lost by returning to the source 12 (approximately 50% of irradiance entering the system). The s-polarized irradiance exits the system 13 (also approximately 50% of irradiance entering the system). For hollow metal coated cube-corners, the polarization state at the detector is nearly the same as the source polarization state, resulting in little irradiance present at the detector. This is shown in FIG. 3, where it is illustrated that the cube-corner with metallic coating 14 produces p-polarized irradiance 15 exiting the metallic coating cube-corner 14 and the p-polarized irradiance exiting the system 16 with 100% of the irradiance returning to the source (lost). Some systems place a quarter-wave linear retarder in the optical path, as is shown in FIG. 4. In FIG. 4, p-polarized irradiance 7 enters the PBS 8 with the entire entering irradiance exiting the PBS 9 and entering a quarter wave linear retarder (“QWLR”) with fast axis oriented at 45 degrees to incident polarization 17. The irradiance exits the QWLR as right circular polarized light incident on the cube corner with metallic coatings 19. The irradiance then exits the cube corner 19 as left circular polarized irradiance 20 and re-enters the QWLR 17. The irradiance then exits the QWLR 17 in s-polarized form 21. As the irradiance is s-polarized, 100% of the entering irradiance exits the system and none is lost by returning to the source. Thus, in principle, adding a quarter-wave linear retarder allows 100% linear polarization coupling with the metal coated cube corner. However this adds cost and complexity to the system, since the waveplate must be carefully aligned. In practice, systems have not approached 80% polarization coupling.